The present invention relates to a fluidized-bed device with oxygen-enriched oxidizer. Fluidized-bed device here means both a circulating fluidized-bed combustion chamber and a dense fluidized-bed combustion chamber.
The development of new regulations limiting greenhouse gas emissions, including carbon dioxide in fossil fuel power plants, is leading to the implementation of technologies for capturing the carbon dioxide present in air combustion flue gases.
The frequently used carbon dioxide capture technology consists in scrubbing the flue gases diluted with nitrogen from the combustion air using solvents which absorb the carbon dioxide and then restore it as a concentrated carbon dioxide gas stream after regenerating the solvent by heating.
This method consumes a considerable amount of energy and substantially reduces the efficiency of fossil fuel power plants, namely reducing the efficiency by more than 15 percentage points.
U.S. Pat. No. 4,498,289 and U.S. Pat. No. 5,175,995 teach the use of oxygen as oxidizer instead of air in power generation boilers.
The advantage of using oxygen as oxidizer is the reduction, to the point of elimination, of the nitrogen ballast diluting the carbon dioxide present in the flue gases and originating from the nitrogen present in the combustion air, and the substantial reduction of the size of the necessary equipment.
The application of this principle to a circulating fluidized-bed boiler is disclosed in U.S. Pat. No. 6,505,567. According to this document, a fluidized-bed combustion chamber or steam generator comprises means for introducing substantially pure oxygen into the generator.
The advantage of the circulating fluidized-bed technique is that it allows the extraction of the heat on the circulating solids loop and the maintenance of a low combustion temperature, independently of the oxygen content of the oxidizer. Hence this technique is particularly attractive and serves to maximize the fraction of oxygen in the oxidizer and to minimize the size of the boiler which depends directly on the flow rate of flue gases produced.
However, in this prior document, no details are provided concerning these injection means.
In fact, this oxygen injection into the combustion chamber is particularly difficult and the creation of local hot spots is liable to cause caking of the bed and destruction of the injection nozzles. In fact, the medium of a circulating fluidized bed is not a perfectly stirred medium in practice, despite the high solids concentration which guarantees local turbulence.
Furthermore, this injection must ensure an appropriate oxygen distribution over the cross section of the combustion chamber. The contacting and the reaction of the fuel introduced into the bed with the oxidizer is essential for the satisfactory progress of the reactions and, in particular, to minimize the formation of nitrogen oxides and maximize the conversion of desulfurization limestone injected into the combustion chamber.
U.S. Pat. No. 4,628,831 describes a fluidized-bed device provided with a hearth equipped with gas mixture feeds called primary feeds, said gas mixture containing oxygen, said hearth being provided with a network of two types of primary feed nozzles, a first type of nozzle injecting the gas mixture at a first level close to the hearth and a second type of nozzle injecting the gas mixture at a second level above the first level.
According to that document, the gas mixture must be prepared upstream in large volumes. This implies a large overall size of the boiler.
Added to this are safety problems inherent in the presence of large volumes of oxygen-concentrated gases.